Statistically Toleranced Drawing Feature Dimensions - Target Value and Range

M

martin elliott

#1
Statistically Toleranced Dimensions

We are early days with a new end user and amongst the numerous customer background documents is the requirement for Key Characteristics to be controlled in a slightly different way to our norm.

Fortunately there are no Keys specified up to date, but I thought I had better understand it for the, hopeful, future business. But before I go back to them and look ignorant, I thought I would rather look ignorant to the Covers as you are bound to know the answer or if I am on the right track!

The requirement is that the “Target Value and Acceptable Range should be expressed in terms of a Mean, Standard Deviation and acceptable Sigma Level requirements”.

My best interpretation is:-

Assume a maximum Sigma level could be Design Tolerance/12
Assuming a 4 sigma Level, 1.33 then the Mean would need to be within 2 sigma of target.
Assume a feature was designed to be at 75.00 plus/minus 0.25mm.
Maximum Sigma = 0.50/12 = 0.0417.

So the actual drawing dimension feature would need to be shown as:-

Mean 74.917-75.083
Sigma Max 0.0417.
Cpk Min 1.33

Comments?

Martin in England
Getting Older but no wiser
 
Elsmar Forum Sponsor
B

Bill Ryan - 2007

#2
Statistical Tolerancing is a relatively new concept. I have not run into it on a part drawing yet. My understanding of the concept is that a feature may be "statistically toleranced" if it is produced using statistical process control methods. It is a less restrictive tolerance that is applied only after proving manufacturing capability. If the manufacturer can not prove capability, a more restrictive tolerance must be used. I think both tolerances need to be shown on the drawing. As I mentioned, I have no experience with this concept but do remember it being discussed in a couple of GD&T classes I attended.
 

Statistical Steven

Statistician
Staff member
Super Moderator
#3
It makes perfect sense, and it is how all specifications should be developed. It works backwards by using the process variability to determine what specifications yield an capable process. Of course, the statistical tolerances need to be checked against customer expectations.

martin elliott said:
Statistically Toleranced Dimensions

We are early days with a new end user and amongst the numerous customer background documents is the requirement for Key Characteristics to be controlled in a slightly different way to our norm.

Fortunately there are no Keys specified up to date, but I thought I had better understand it for the, hopeful, future business. But before I go back to them and look ignorant, I thought I would rather look ignorant to the Covers as you are bound to know the answer or if I am on the right track!

The requirement is that the “Target Value and Acceptable Range should be expressed in terms of a Mean, Standard Deviation and acceptable Sigma Level requirements”.

My best interpretation is:-

Assume a maximum Sigma level could be Design Tolerance/12
Assuming a 4 sigma Level, 1.33 then the Mean would need to be within 2 sigma of target.
Assume a feature was designed to be at 75.00 plus/minus 0.25mm.
Maximum Sigma = 0.50/12 = 0.0417.

So the actual drawing dimension feature would need to be shown as:-

Mean 74.917-75.083
Sigma Max 0.0417.
Cpk Min 1.33

Comments?

Martin in England
Getting Older but no wiser
 
B

Bill Ryan - 2007

#4
As more of the cobwebs melt away and a few of the synapses start clicking a bit more like the old days :rolleyes: , I seem to remember an instructor saying this concept would probably be a tough sell for a while because most "older" (even a lot of "newer"?) Design Engineers just don't grasp GD&T very well. In other words, they peformed their stackup studies and then divvied the results out to the two, three, etc. components making up an assembly. Their "safety fudge factor" in most cases is inviolate.
 

Jim Wynne

Staff member
Admin
#5
martin elliott said:
Statistically Toleranced Dimensions

We are early days with a new end user and amongst the numerous customer background documents is the requirement for Key Characteristics to be controlled in a slightly different way to our norm.

Fortunately there are no Keys specified up to date, but I thought I had better understand it for the, hopeful, future business. But before I go back to them and look ignorant, I thought I would rather look ignorant to the Covers as you are bound to know the answer or if I am on the right track!

The requirement is that the “Target Value and Acceptable Range should be expressed in terms of a Mean, Standard Deviation and acceptable Sigma Level requirements”.

My best interpretation is:-

Assume a maximum Sigma level could be Design Tolerance/12
Assuming a 4 sigma Level, 1.33 then the Mean would need to be within 2 sigma of target.
Assume a feature was designed to be at 75.00 plus/minus 0.25mm.
Maximum Sigma = 0.50/12 = 0.0417.

So the actual drawing dimension feature would need to be shown as:-

Mean 74.917-75.083
Sigma Max 0.0417.
Cpk Min 1.33

Comments?

Martin in England
Getting Older but no wiser
It's not real clear what the customer is asking for (which is why you're here, I guess). If the intent was to use statistical tolerancing as it's described in Y14.5M, then the customer has apparently misunderstood the requirements. I've attached the relevant section of the standard here (fair use applies, I think) for reference. Statistical tolerancing is intended to be applied in stackup situations, not in unilateral control of key characteristics. The standard describes the language to be used on drawings when ST is applied.

As for what the customer is asking for, it doesn't seem to make much sense to me; what's accomplished that just specifying a minimum Cpk level wouldn't? What is meant by "Target values...should be expressed in terms of a mean..."? How is that different from a nominal value? What is the customer asking when he says "...standard deviation and acceptable Sigma level"? (Emphasis added) What does "Sigma level" mean in this context?

Anu number of Covers might be able to make educated guesses as to what was intended, but to be honest, the requirements as expressed here are unintelligible. When it's not possible to use the specifications to interpret what the customer wants, the only prudent recourse is to ask the customer for clarification.
 

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M

martin elliott

#6
Thank you all very much for efforts.

As Jim says, it was not clear to me what the customer means and that's why I was asking for comment. To add insult to injury, looking further into his tome, I suspect some of his other requirements are less than clear, but the customer is always right;) .

In answer to the question on capability and sigma, I thought he was asking what Cpk our process was to achieve long term.

Thank you again for your imput and it looks like I will have to appear ignorant to the customer if I am to get to the bottom of it :eek: .

Martin in England
Getting very much Older but no wiser
 
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